The removal of undesired nitrates, injurious to health, from waters by microorganisms (denitrification) with the use of gaseous reducing agents is known. At present, hydrogen and methane are used. In the case of methane, however the use was restricted to the laboratory scale.
As compared with denitrification by means of liquid reducing agents such as, for example, methanol, ethanol, organic waste products, excess sludge and the like, a gas generally has the advantage that overdosage and the consequent residual pollution need not be feared. The disadvantage of hydrogen is, however, the high operating and investment cost, since the denitrification process must be carried out under a pressure of about 5 bar gauge because of the low solubility (for example 1.7 g of H.sub.2 /1 at 10.degree. C.).
Although sufficient solubility applies in the case of methane, whose suitability in principle has been proved in laboratory experiments (M. Werner, "Denitrifikation unter besonderer Berucksichtigung externer Kohlenstoffquellen" [Denitrification with special consideration of external carbon sources], publication of the Zentrum fur Abfallforschung [Waste Research Centre] of Brunswick Technical University, publisher: Kayser, Albers, issue No. 3, 1988), the low, uneconomical denitrification rate proved to be a disadvantage. According to present knowledge, the limiting step of the two-stage reaction is the conversion of the methane into methanol by means of oxygen in the first step. The use of methane has therefore so far been restricted to the laboratory scale.